911sc speedo

[hellblau82]

Hi guys

Just trying to distguish if my speedo is any good in my car..

Can anyone tell me if you should see signs of life like you do on the other gauges when you turn the accessorys on the ignition.

Should the needle flutter or anything?

Car is in the garage and can't drive it out just yet but I can see all the other gauges activate when I switch the ignition on.

I'm assuming the SPEEDO doesn't actually start moving until the vehicle is moving?

I can't see the needle move or anything like I do on all the other gauges. Will test drive it tomorrow I guess

[universeman]

IIRC speedo needle doesn't move on ignition.

[dap930]

If you want to test it, jack up the rear and spin a rear tire with the key on to see if you get needle movement. There is a plate inside the transmission with 8 small magnets on it. Outside the transmission there is sensor puck which is a reed switch. When a magnet passes by the switch, the contacts close is presence of magnetic flux which produces a pulse to the speedometer. There 8 pulses per tire revolution. The sensor puck is connected to the speedometer in transmission tunnel where the shift coupler is.

[Walt Fricke]

Or you can pull the speedo out of the dash, and clip a wire to the sensor terminal. With the ignition on, tap rapidly the other end of that wire to a chassis ground (the case of the speedo should do). You should see the needle move and indicate a few MPH depending on how fast you can tap.

If this doesn't work with the sensor wire still attached to the speedo, take the sensor wire off so only your wire is attached and try again, in case the differential has a magnet lined up with the reed switch in such a way that the system is grounded already.

If this doesn't work, there is an issue with the speedo. But what you have observed is no indication at all of a speedo problem. The speedo circuit doesn't work like the oil pressure and other indicators do. They require a resistance from the sensors to keep them at zero when the key is on and the instruments are powered, so they jump from a resting position to zero when you turn the key on.

Now if the alternator warning light does not come on when you turn the key on before starting, you may have a problem with that circuit. But not the speedo.

[hellblau82]

Originally Posted by Walt Fricke

Or you can pull the speedo out of the dash, and clip a wire to the sensor terminal. With the ignition on, tap rapidly the other end of that wire to a chassis ground (the case of the speedo should do). You should see the needle move and indicate a few MPH depending on how fast you can tap.

If this doesn't work with the sensor wire still attached to the speedo, take the sensor wire off so only your wire is attached and try again, in case the differential has a magnet lined up with the reed switch in such a way that the system is grounded already.

If this doesn't work, there is an issue with the speedo. But what you have observed is no indication at all of a speedo problem. The speedo circuit doesn't work like the oil pressure and other indicators do. They require a resistance from the sensors to keep them at zero when the key is on and the instruments are powered, so they jump from a resting position to zero when you turn the key on.

Now if the alternator warning light does not come on when you turn the key on before starting, you may have a problem with that circuit. But not the speedo.

Ok I shall give that a go...
The terminals i have are

+ 12v
- ground /sensor wire1
31b ??? Sensor wire 2
A. ???? - not sure what this is for

It looks like I have 1 cable that spits into 2 connectors

[Walt Fricke]

Pull the middle brown wire with red stripe and put your alligator clip on the its speedo terminal. Or pull both those brown wires with red stripes, and use the alligator clip wire to "tap" the two together.

For some reason Porsche didn't just ground the reed switch on the transmission, or somewhere else nearby. Instead it ran both wires from the sensor all the way back to the speedometer. There one of them is connected to ground, and the other is the "signal" wire. And it doesn't matter which is which, which is why both wires have the same wire color code.